As an important means for the study of molecular structures, intermolecular interactions and the like, nuclear magnetic resonance (NMR), has been widely applied to the material sciences, physics, chemistry, biology, and medical science. However, NMR is fundamentally limited in sensitivity. Dynamic nuclear polarization (DNP) technology based on electron-nuclear polarization transfer provides an important approach to the enhancement of the NMR. DNP is an electron-nuclear double resonance technology, where electron spin is controlled with microwave to transfer the high spin polarization of unpaired electrons (free radicals) to a nucleus for enhancing the spin polarization of the nucleus. Since the gyromagnetic ratio (or polarization) of the electron is 660 times (2600 times for 13C) that of proton, the maximal DNP enhancement of the proton is 660 times (2600 times for 13C) if the polarization of the electron is completely transferred to a corresponding nucleus.
An electron-nuclear double resonance system typically includes three magnetic fields, with one being a main magnetic field B0, and the other two being a radio-frequency field B1 generated by a radio-frequency coil and a microwave field B2 generated by a microwave resonator respectively. B1 is used for exciting nuclear magnetic resonance, and B2 is used for exciting electron spin resonance.
The double resonance resonator is an important component of the dynamic nuclear polarization spectrometer. Traditional metallic resonant cavity has been widely used due to its high Q value. However, the high Q value of the traditional metallic resonant cavity is not suitable for the excitation of a pulse microwave, and meanwhile, the traditional metallic resonant cavity can't be used for a liquid sample having a high dielectric constant due to large size and poor fill factor and poor distribution of a microwave magnetic field. Although certain problems can be solved by a newly developed loop-gap resonator, the radio-frequency coil taking the form of a solenoid is generally placed outside the loop-gap resonator, and meanwhile, it plays the role of shielding the loop-gap resonator. To reduce the shielding effect of the loop-gap resonator on the radio-frequency field, the loop-gap resonator is generally manufactured by performing silver plating on a quartz glass surface. Since the radio-frequency coil is required to play the role of shielding the loop-gap resonator, it is desired that the diameter of the radio-frequency coil be more than 2 times that of the loop-gap resonator to obtain a microwave resonance mode as desired, i.e., a better field distribution, thereby greatly reducing the fill factor of the radio-frequency coil. According to theoretical deduction, the detection sensitivity of a system is in direct proportion to the fill factor of the coil, and the practice above will greatly reduce the detection sensitivity of the system.